1. Technical Field
The present application relates to liquid level sensor systems. More specifically, the present application relates to the rejection of interference which adversely affects the operation of a capacitive liquid level sensor (LLS) system.
2. Description of Related Art
Withdrawing and dispensing precise volumes of a liquid without contaminating the liquid is an important part of many clinical applications and laboratory tests. While attempts to address these needs manually have failed to provide the needed accuracy and purity, a number of automated or semi-automated liquid-measuring systems are presently being used to gauge more precisely the small liquid volumes that need to be withdrawn and/or dispensed.
One of the more popular types of liquid-measuring systems uses a motor-controlled pipette-like probe to aspirate or dispense a desired amount of fluid from or into a container. The probe is movably mounted over the container and, using a precision-controlled motor, is vertically (z-axis) lowered into the container until the tip of the probe reaches a desired level below or above the upper surface of the liquid (the meniscus). A desired amount of liquid is then withdrawn from or dispensed into the container. Such systems have been designed to: minimize/reduce cross-contamination between the contents of different containers, avoid splashing of the liquid during the aspiration (or deposition) process, and minimize/reduce the portion of the probe that must be washed.
In many instances, the automated or semi-automated liquid-measuring system does not know beforehand the level of fluid contained within the container. Nonetheless, the pipette-like probe must be lowered to a certain position with the expectation that the probe has been precisely positioned with respect to the fluid level. Several systems control the position of the probe tip without previously knowing the upper level of the liquid in the container by sensing for the upper level of the liquid in the container as the probe is being lowered. For example, a measurement can be made of some electrical phenomena associated with a change in the capacitance between the probe and the liquid in the container as the tip of the probe approaches the liquid. This measurement may identify a liquid sense event (for example, penetrating the meniscus or withdrawing from the meniscus) when the capacitance between the probe and the liquid reflects a change in voltage level that is greater than a threshold reference level.
A well known system and technique for capacitive-based sensing of liquid level in a z-axis controlled liquid-measuring system is taught by U.S. Pat. No. 5,365,783, to Ronald A. Zweifel (the “Zweifel system”), the disclosure of which is hereby incorporated by reference.
In some existing systems, the labware can collect and store static electricity. As the probe is moved closer and closer to the container, one or more static discharge events may occur between the probe and container during probe movement. These static discharge events can cause an instantaneous change in a signal indicative of the measured capacitance which can incorrectly be detected by the system as a liquid event (for example, a false positive indication that the probe is in the liquid when in fact it is still positioned above the liquid). Second, the laboratories where these systems are commonly used are typically illuminated using fluorescent light fixtures. The electronic ballasts used by such fixtures emit high frequency electromagnetic radiation. Probes can thus act as an antenna with respect to such radiation, and the corresponding noise signal from that antenna-captured radiation can adversely affect a signal indicative of the measured capacitance and can cause an incorrect detection of a liquid event.